Numerical method optimizes aero-assisted orbital interceptions

Tony Beresford (
Sat, 04 Dec 1999 11:57:29 +1030

[ this is cross posted from ASTRO. My apologies to ASTRO members on seesat,
but the topic does influence future satellite operations -ACB]
News Bureau
University of Illinois at Urbana-Champaign
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Champaign, IL 61820-6219
(217) 333-1085   fax (217) 244-0161

James E. Kloeppel, Physical Sciences Editor
(217) 244-1073

December 1999

Numerical method optimizes aero-assisted orbital interceptions 

CHAMPAIGN, Ill. -- Future spacecraft may use a planet's atmosphere
to generate aerodynamic forces that modify the crafts' orbits without
using fuel. University of Illinois researchers have developed a numerical
technique that can optimize the paths of these aero-assisted orbital
transfer vehicles. 

"Today's spacecraft use propellant-powered thrusters to move from one
orbit to another," said Bruce Conway, a professor of aeronautical and
astronautical engineering. "But each pound of fuel carried aloft means
a corresponding reduction in the weight of the mission payload. A
next-generation spacecraft may switch orbits at a substantial fuel
savings by dipping into the atmosphere, generating aerodynamic lift
and drag on airplane-like control surfaces, and then climbing to a
new orbit." 

The concept is similar to aerodynamic braking, a technique that uses a
planet's atmosphere to reduce the speed of a space vehicle. Aerodynamic
braking was used successfully for the Mars Global Surveyor mission,
currently in orbit around the red planet. 

"It would have been far too costly to send that spacecraft to Mars with
all the necessary fuel to place it in the desired orbit through a prolonged
'burn,' " Conway said. "Instead, the mission planners placed the spacecraft
in a more accessible but highly elliptical orbit, and used aerodynamic
braking to slowly circularize the orbit. The procedure took several
months to complete, but required little fuel." 

In much the same fashion, aero-assisted orbital transfer vehicles could
use a planet's atmosphere to change the altitude, shape or plane of their
orbits. Applications in Earth orbit include atmospheric sampling,
satellite repair missions, surveillance and missile interception. 

In a paper published in the September-October issue of the Journal of
Guidance, Control, and Dynamics, Conway and graduate student Kazuhiro
Horie applied a new numerical method they developed at the university. 

"Optimal trajectories were found for the interception of a target
in low-Earth orbit by a vehicle initially in a higher orbit, using
aero-assist," Conway said. "We obtained solutions for a wide range of
target orbit inclinations and constraints, such as maximum allowed
heating rates. We also found that our method could solve problems
with demanding constraints that conventional methods could not." 

Using aero-assist made some interceptions possible that otherwise
were infeasible because the spacecraft carried insufficient fuel. But

an interesting and non-intuitive result was that in some cases, even
if the spacecraft had enough fuel to intercept the target without
entering the atmosphere, using a combination of conventional
propulsion and aero-assist yielded a quicker interception. 

"Unlike other methods, our technique is particularly well-suited for
solving problems where many complicated constraints are placed on
the trajectory," Conway said. "Our numerical method looks first for
any trajectory that starts with the given conditions and satisfies
the desired goal, then it optimizes that trajectory for the best
possible solution."

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